Hydrophilic microchannel

Sintov, A.C., et al. 2003. Radiofrequency-driven skin microchanneling as a new way for electrically assisted transdermal delivery of hydrophilic drugs. J Control Release 89 311. 

As in the case of normal chromatography both stationary and mobile phases are also required in NLC. On the other hand, in NCE hydrophilic channel walls with improved control over electroosmotic flow are required for better separation of biological samples. Briefly, the separation efficiencies and selec-tivities in NLC and NCE depend on the properties of the microchannels, and, therefore, surface modification of the microchannel is usually necessary to achieve good separation of a variety of analytes. Recently, Muck and Svatos... 

Affinity chromatography of streptavidin was performed on a PET chip. The microchannel was first filled with the dual-modified latex beads (as shown in Figure 6.3). The biotinylated beads were surface-modified with a temperature-sensitive polymer, poly(N-isopropylacrylamide (PNIPAAm, 11 kDa). When the temperature was raised above the lower critical solution temperature (LCST) of PNIPAAm, the beads aggregated and adhered to the channel wall, because of a hydrophilic-to-hydrophobic phase transition. Then streptavidin from a sample solution was captured by these adhered biotinylated beads. Thereafter, when the temperature was reduced below the LCST, the beads dissociated and eluted from the channel wall together with the captured streptavidin [203],... 

Figure 18 UV photopatterning method. The molecular structure of a photocleavable SAM formed on glass surfaces. UV irradiation through masks placed on top of SAM-modified microchannels leads to the production of hydrophilic carboxylate groups in the irradiated regions (Zhao et al, 2001).

The hydrophilic or hydrophobic nature of the wall surface can modify the boundary conditions and introduce a slip condition Choi et a/. [23] used high precision microchannels treated chemically to enhance the hydrophilic and hydrophobic properties of wall surfaces. 

Liquid viscosities have been observed to increase, decrease, and remain constant in microfluidic devices as compared to viscosities in larger systems. ° Deviations from the no-slip boundary condition have been observed to occur at high shear rates. One important deviation from no-slip conditions occurs at moving contact lines, such as when capillary forces pull a liquid into a hydrophilic channel. The point at which the gas, liquid, and solid phases move along the channel wall is in violation of the no-slip boundary condition. Ho and Tai review discrepancies between classical Stokes flow theory and observations of flow in microchannels. No adequate theory is yet available to explain these deviations from classical behavior. ... 

D. C. Tretheway and C. D. Meinhart, Apparent fluid slip at hydrophobic microchannel walls, Phys. Fluids 14, L9-L12 (2002) Chang-Hwan Choi, J. A. Westin, and K. S. Breuer, Apparent slip flows in hydrophilic and hydrophobic microchannels, Phys. Fluids 15, 2897-902 (2003). 

In phase separation two immiscible fluids are physically separated. Microchannels offer the ability to separate phases in an orientation-independent manner, since capillary and surface tension forces are more dominant in these high-surface-area devices. Various microchannel phase separators have been developed to separate organic and aqueous phases for use in unit processes such as solvent extraction or reactions conducted at an aqueous organic interface [185-188]. The approach is to hydrophobize half of the channel with a non-polar agent so that the organic phase is constrained to the hydrophobic half and the aqueous phase to the hydrophilic half Phase separation is simply then a matter of splitting the flow at the hydrophobic-hydrophilic junction of the flow. 

A study conducted by Zhao et al. created intricate patterns of hydrophilic and hydrophobic regions down the microchannel to improve extraction [205]. This was achieved by laying down photocleavable SAM (self-assembled monolayer) and exposing with UV light to create exposed hydrophilic regions. The unexposed regions remained hydrophobic. If pressure is controlled properly, once patterned, the aqueous phase will remain separated from the organic phase. 

Scheiff, F., Mendorf, M., Agar, D., Reis, N., Mackley, M. (2011). The separation of immiscible liquid slugs within plastic microchannels using a metallic hydrophilic sidestream. Lab on a Chip, 11, 1022-1029. 

Choi et al. [1] presented the slip velocity and slip length of hydrophilic and hydrophobic microchannels (1 and 2 pm depth) based oti the flow rate and pressure drop measurements. Sample results from their study are compiled in Fig. 2. The flow rate for hydrophobic surface is higher than that compared to the hydrophiUc surface. The corresponding slip velocity and sUp length for hydrophobic surface is also higher than that of the hydrophilic surface. The slip velocity and sUp length increases with strain rate. 

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